1. Field of the Invention
The present invention relates to the production of crystalline aluminosilicate ZSM-5, and more specifically, to improved methods for rapdily and efficiently producing ZMS-5 crystals from the zeolite forming solution.
2. Description of the Prior Art
The crystalline aluminosilicate zeolite known as ZSM-5 is particularly described in U.S. Pat. No. 3,702,886, the disclosure of which is incorporated herein by reference. ZSM-5 crystalline aluminosilicate is characterized by a silica-to-alumina mole ratio of greater than 5 and more precisely in the anhydrous state by the general formula: EQU [0.9.+-.0.2M.sub.2 /.sub.n O:Al.sub.2 O.sub.3 :&gt;5SiO.sub.2 ]
wherein M is selected from the group consisting of a mixture of alkali metal cations and organo ammonium cations, particularly a mixture of sodium and tetraalkyl ammonium cations, the alkyl groups of which preferably contain 2 to 5 carbon atoms. The term "anhydrous" as used in the above context means that molecular water is not included in the formula. In general, the mole ratio of SiO.sub.2 to Al.sub.2 O.sub.3 for a ZSM-5 zeolite can vary rapidly. For example, ZSM-5 zeolites can be aluminum-free in which the ZSM-5 is formed from an alkali mixture of silica containing only impurities of aluminum. All zeolites characterized as ZSM-5, however, will have the characterisitic X-ray diffraction pattern set forth in U.S. Pat. No. 3,702,886 regardless of the aluminum content of the zeolite.
Seeding as a means for inducing crystallization is a very old technique. In the art of zeolite manufacture, various patents describe the use of seeding to induce the rapid crystallization of zeolites. Various patents describing the manufacture of zeolite crystals by seeding with a zeolite include: United Kingdom Pat. No. 1,297,256, in making ZSM-4; U.S. Pat. No. 3,247,194, in making ZK-5; U.S. Pat. No. 3,733,391, in making faujasite; and U.S. Pat. No. 4,007,253, in making faujasite in which the seed is not the same as the product. Patents disclosing the formation of zeolites by seeding with other aluminosilicates include: United Kingdom Pat. No. 1,117,568, in making ZSM-4; United Kingdom Pat. No. 1,160,463, in making faujasite; U.S. Pat. No. 3,578,398, in making a zeolite similar to offretite; and U.S. Pat. No. 3,947,482, in making various zeolites. It is to be understood that the mentioned preceding patents are not an exhaustive list of all patents which discuss forming zeolite crystals by seeding.
U.S. Pat. No. 4,275,047 discloses a method of making a zeolite of the ZSM-5 family by reacting an alkali mixture comprising an alumina source, a silica source and alkali metal ions in the presence of a seed quantity of a zeolite having a pore diameter in the range of 5.8 to 7.0 Angstrom units until the required zeolite has been formed and recovering the product zeolite. Zeolites that can be used as seeds include members of the ZSM-5 family including ZSM-5, ZSM-8, ZSM-11 and ZSM-12. Other zeolites include those having pore diameters in the above-stated range, in particular, zeolite nu-1 described in U.S. Pat. No. 4,060,590 and zeolite FU-1 described in U.S. Pat. No. 4,209,498. Other examples of zeolites useful as set forth in the patent include brewsterite, clinoptilolite, dachiardite, epistilbite, ferrierite, heulandite and stilbite, as well as ZSM-21, ZSM-35 and ZSM-38.